Overload protection for a disc cutterbar

ABSTRACT

A disc cutterbar having a two-piece mounting hub, one piece rotatably driven and the other supporting a knife for severing standing crop material, with an epoxy layer bonding the two pieces together and forming a shear device therebetween. A brake is associated with the knife-supporting piece whereby upon failure of said shear device, the knife-supporting piece is stopped from rotating within one revolution.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application relates to U.S. patent application Ser. No. ______ entitled “Brake for a Disc Cutterbar” in the name of Timothy J. Kraus, filed on the same date as this application.

FIELD OF THE INVENTION

[0002] The present invention relates generally to mechanisms for protecting mechanical drive components from overloads, and more particularly relates to a shear device and brake coupled between components of an agricultural disc mower that protects the various components of the mower in the event a cutterhead strikes an object and creates an overload condition.

BACKGROUND OF THE INVENTION

[0003] Typical disc cutterbars used in agriculture include an elongated housing containing a train of meshed idler and drive spur gears, or a main power shaft coupled by respective bevel gear sets, for delivering power to respective drive shafts for cutterheads spaced along the length of the cutterbar. The cutterheads each comprise a cutting disc including diametrically opposed cutting blades (though configurations with three or more blades are known) and having a hub coupled to an upper end of a drive shaft, the lower end of the drive shaft carrying a spur gear in the case where a train of meshed spur gears is used for delivering power, and carrying a bevel gear of a given one of the bevel gear sets in the case where a main power shaft is used. In either case, bearings are used to support the various shafts. The cutterheads are rotated at a relatively fast speed making the drive components, such as gears, bearings and shafts, vulnerable to damage in the event that the unit strikes a foreign object. For background information on the structure and operation of some typical disc cutterbars, reference is made to U.S. Pat. No. 4,815,262, issued to E. E. Koch and F. F. Voler, the descriptive portions thereof being incorporated herein in full by reference.

[0004] In order to minimize the extent of such possible damage to the drive components, it is known to incorporate a shear device somewhere in the drive of each unit which will “fail” upon a predetermined overload being imposed on the device. As used herein with reference to shear devices, the terms “fail” or “failing” are intended to cover the actual function of such devices, i.e., shearing, fracturing, breaking and the like. Several different such shear devices and arrangements are shown in U.S. Pat. Nos. 4,999,981, 4,497,161 and 5,715,662.

[0005] The '981 patent shows a shear mechanism that comprises a shaft with a weakened portion created by a cut groove, or break zone 41 (seen, for example, in FIG. 3 thereof in driven shaft 20. Upon overload, the shaft breaks at zone 41 that is located outside the support bearing such that there is a clean and complete break in the shaft. This structure is intended to eliminate the input of kinetic energy to the cutterhead after failure of the shear mechanism, thereby eliminating damage to the drive system and gearing. While this structure may in fact eliminate the input of further kinetic energy, it does not stop rotation of the cutterhead or prevent the damage that continued rotation would generate.

[0006] A somewhat different shear mechanism is disclosed in FIGS. 2 and 3 of the '161 patent. Cutting disc 3 is connected by a series of shear bolts 26 to the vertical shaft 8. Upon impact of the cutterhead with an obstruction, the shear bolts fail, stopping the input of rotational force to the cutterhead. FIG. 4 shows a slightly different embodiment where a resilient cover plate 28 depresses balls 30 arranged in holes of the disc 3 and fitting into recesses 31 of the disc 27. An overload impact is intended to cause balls 30 to snap out of the recesses 31 so that the direct rotary joint between shaft 8 and cutting disc 3 is interrupted. It is stated that the connection can be re-established by continuing to rotate disc 3 with respect to the disc 27 so that the balls 30 again snap into the recesses 31. The embodiments set forth in this patent exhibit the same shortcomings as seen in the '981 patent, i.e., standard shear mechanisms do not stop rotation of the cutterhead, and thus do not prevent additional damage thereby encountered.

[0007] The shear mechanisms shown in the '662 patent each employ shearable splines. A first embodiment the shear device is in the form of either a collar or clamping member having internal splines received on a splined upper end of the drive shaft and having shearable cylindrical drive lugs engaged with complementary shaped openings provided in an upper surface of a disk hub. Referring more specifically to FIG. 2 thereof, the upper end of drive shaft 26 has a splined section 86. Shear collar 88 establishes a drive connection between shaft 2 i 6 and hub 80. The collar 88 includes internal splines 90 engaged with the splined section 86 of shaft 26 just above hub 80. Shearable cylindrical drive lugs 92 project downwardly from the bottom of collar 88 and are received in complementary holes 94 in hub 80. An overload situation causes the lugs 92 to shear and the continuing transfer of rotational power to cease. FIGS. 4 through 6 shows another embodiment where shaft 34 has a splined upper end section 110. Instead of a shear collar, a shear device in the form of a cap-like clamping member 114 is used for transferring torque from shaft 34 to hub 80. Clamping member 114 has an annular lower portion 116 provided with interior splines 118 engaged with the splined section 110 of shaft 34. A plurality of shearable lugs 120 extend downward from lower portion 116 and are received in complementary shaped cylindrical openings 94 in hub 80, whereby torque is transferred from shaft 34 to hub 80. Again, when an overload occurs, lugs 120 shear, and torque is no longer transmitted. The final embodiment shown in the '662 patent is shown in FIGS. 7 through 9. Instead of a disk hub 80, a disk hub 127 is used which has a central splined opening 128 disposed in spaced concentric relationship t the splined upper end section 110 of shaft 34. A ring-like shear insert 130 is received on the upper end of the drive shaft 34 and has inner splines 132 engaged with the splined upper end section of the shaft and has outer splines 134 engaged with the splined opening 128 of hub 126. Splines 132 are designed to shear upon overload.

[0008] Similar to the other prior art discussed above, the embodiments of the '662 patent do not stop the cutterhead from rotating, even after power is cut off by a shear device. The third embodiment shown in this patent exhibits an additional shortcoming in that upon failure of the shearable splines, the broken pieces tend to become temporarily “jammed” in among the other parts and components, resulting in even further, though short lived, torque to be transferred, and the resultant additional damage to the cutterheads.

[0009] Particularly in its preferred embodiment, the instant invention overcomes the drawbacks and shortcomings of the prior art. A two-piece hub design, with an epoxy shear surface therebetween will fail with substantially no residual transfer of torque. In fact, this shear mechanism results in no discernable residue, and is easily repaired by merely adding new epoxy. Upon failure, the two-piece hub, one of which is driven directly by a drive shaft, separates and the non-driven portion engages a braking mechanism that stops rotation of the cutterhead, i.e., the non-driven portion of the hub.

[0010] While the concept of a shear mechanism other than shear pins, lugs or bolts is not new, the use of a frangible layer such as epoxy in the environment of a cutterbar is new and provides advantages in overcoming the problems and shortcomings of the prior art as discussed above. In order to limit the damage to a cutterbar in an overload situation, two characteristics are pursued—a quick, clean disengagement of the driven elements, and a rapid end of rotation of the cutterhead. The epoxy layer provides the first, and the novel brake the second. For non-traditional shear mechanisms, attention is directed to U.S. Pat. Nos. 2,056,785 (rubber), 3,064,454 (solder, glass, and other fracturable and fusible materials), and 3,521,464 (plastic).

SUMMARY OF THE INVENTION

[0011] Accordingly, one object of the present invention is to provide a brake, operating in conjunction with a shear mechanism, in a mechanical drive train for an agricultural cutterbar that will not only stop the transfer of power along the drive train in the event of overload, but also stop rotation of the non-driven components before further damage can occur.

[0012] A further object of the present invention is to provide a novel shear device between hub components of a cutterhead.

[0013] Yet another object of the present invention is to provide a disc cutterbar with multiple cutterheads, each comprising a drive shaft connected to an inner hub which is connected to an outer hub via a shear mechanism. A novel brake mechanism is triggered upon failure of the shear mechanism, stopping rotation of the lower hub and cutterhead.

[0014] It is yet a further object of this invention to provide an improved disc cutterbar that is relatively durable in construction, inexpensive of manufacture, carefree of maintenance, easy to assemble, simple and effective in use, and less likely than prior art cutterbars to sustain costly damage upon contact with a fixed object.

[0015] These and other objects, features and advantages are accomplished according to the instant invention by providing a disc cutterbar having a two-piece mounting hub, one piece rotatably driven and the other supporting a knife for severing standing crop material, with an epoxy layer bonding the two pieces members together and forming a shear device therebetween. A brake is associated with the knife-supporting piece whereby upon failure of said shear device, the knife-supporting piece is stopped from rotating within one revolution.

DESCRIPTION OF THE DRAWINGS

[0016] The advantages of this invention will be apparent upon consideration of the following detailed disclosure of the invention, especially when taken in conjunction with the accompanying drawings wherein:

[0017]FIG. 1 is a top plan view of a disc mower mounted on the three-point hitch of a tractor, the disc mower having a modular disc cutterbar incorporating the principles of the instant invention, the rotational path of the individual disc members being shown in phantom, the disc mower being one of the configurations in which the improved disc cutterbar of the instant invention can be utilized;

[0018]FIG. 2 is an enlarged top plan view of a central portion of the assembled modular disc cutterbar depicting two cutterhead modules and an interstitial spacer module, portions of the spacer modules on opposite sides of the cutterhead modules being broken away and the disc members being removed for clarity;

[0019]FIG. 3 is a cross-sectional view of the cutterhead module taken along line 3-3 of FIG. 1;

[0020]FIG. 4 is an enlarged view of a portion of FIG. 3;

[0021]FIG. 5 is a view similar to FIG. 4, showing an exaggerated view of the gap separating inner hub 42 and outer hub 43 after the shear mechanism has failed and the locking blocks have engaged;

[0022]FIG. 6 is an exploded cross-sectional view of the hubs, locking blocks and springs making up significant components of the instant invention;

[0023]FIG. 7 is a top plan view of the lower locking block taken along line 7-7 of FIG. 6;

[0024]FIG. 8 is a bottom plan view of the outer hub and integral upper locking block taken along line 8-8 of FIG. 6;

[0025]FIG. 9 is an exploded view somewhat rotated showing the relationship between the inner hub, the outer hub and the lower locking block;

[0026]FIG. 10 is a cross-sectional view of an alternative embodiment of the brake device taken along line 10-10 of FIG. 11; and

[0027]FIG. 11 is a view of an alternative locking block taken along line 11-11 of FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0028] Referring now to the drawings and particularly to FIG. 1, a modular disc cutterbar incorporating the principles of the instant invention can best be seen in a configuration in which the disc cutterbar is conventionally utilized. For a more detailed description of a conventional modular disc cutterbar and various configurations thereof reference is made to U.S. Pat. No. 5,996,323. The disclosure in that patent is hereby incorporated herein in its entirety by reference.

[0029] Cutterbar 30 is mounted in a disc mower 10 having a support frame 11 connected to the three-point hitch mechanism 3 of a tractor T on which the mower 10 is carried in a conventional manner. The disc mower 10 receives operative power from the conventional tractor power take-off shaft 5. The mower drive mechanism 15 receives the rotational power from shaft 5 and transfers the rotational power to a gearbox 17, which in turn transfers the rotational power to the cutterbar drive mechanism.

[0030] An alternative configuration for the disc cutterbar would be to incorporate the cutterbar in a disc mower-conditioner. This well-known configuration is shown in more detail in U.S. Pat. No. 5,761,890, which is hereby incorporated herein in its entirety by reference. One skilled in the art and knowledgeable about commercial applications of disc cutterbars will readily recognize that there are other specific configurations of cutterbars to which the invention to be disclosed herein will be applicable. Such skilled individual will also readily recognize that the cutterbar need not necessarily be modular in construction.

[0031] Modular cutterbar 30 is formed from alternating cutterhead modules 40 and spacer modules 32. Each cutterhead module 40, as best seen in FIGS. 1-3, includes a hollow cast housing 41 (FIG. 3) having a shape to retain a low profile and to establish an oil reservoir 84 therewithin. As will be discussed in more detail below, the cutterheads 40 are gear driven and assembled in such a manner as to establish a specific timing relationship between adjacent units. More particularly, the cutterheads are arranged such that the knives 82 on adjacent units have overlapping cutting paths, but do not come into contact with each other. Failure to maintain this timed relationship during operation will result in one unit hitting the adjacent unit(s), damaging the cutterheads (and possibly initiating a chain reaction that damages all cutterheads), the drive train of the cutterbar and/or tractor T. In such case, the damage is usually significant.

[0032] Referring now to FIGS. 2-4, it can be seen that each cutterhead module 40 is provided with a forwardly positioned rock guard 65 and a skid shoe 70 that passes beneath cutterhead module 40 for engagement with the surface of the ground. The rock guard 65 has a conventional semi-circular configuration and is mounted to opposing forward mounting arms 35 of spacer module 32 adjacent to the corresponding cutterhead module 40.

[0033] One skid shoe 70 is mounted beneath each cutterhead module 40 to protect the cutterhead module from wear due to engagement with the surface of the ground. Each skid shoe is formed as a generally planar body portion 71 with a mounting tab 73 affixed thereto and projecting upwardly. The body portion 71 is also formed with a forward end 72 that is bent upwardly to engage the corresponding rock guard 65.

[0034] Modular drive mechanism 75, best seen in FIGS. 2 and 3, is fully disclosed in the '323 patent and reference is made thereto for a more complete description.

[0035] Broadly, within each cutterhead unit there is a two-piece hub, one inner hub and one outer hub, normally held together by a shear mechanism. The inner hub is connected to a drive shaft, and the outer hub, including an integral upper locking block, is connected to a rotatable knife support member. Spaced below the outer hub is a fixed lower locking block. These components are arranged such that when a knife engages a solid or fixed object and a shear force generated adequate to cause the shear mechanism to fail, the outer hub rotates freely and drops (preferably under the influence of a downward biased spring) causing the upper and lower locking blocks to engage and the knife support member to cease rotation. By thus preventing the knives from rotating further, damage is prevented to the drive train of the cutterbar and between adjacent cutterhead units. Ideally, the brake will stop rotation in one revolution or less.

[0036] Now, and more specifically, attention is directed to FIG. 9 which shows an exploded perspective view of the inner hub 42, the outer hub 43 and the lower locking block 44. In the preferred embodiment, inner hub 42 is affixed to outer hub 43 by means of a layer of epoxy bonding surfaces 45 and 46 of the two hubs, respectively. By controlling the size of the bonded surface area of the two hubs, and knowing the shear strength of the epoxy, a specific shear point or force can be calculated so that failure will occur at the desired point and upon a specific impact. Outer hub 43 includes an upper locking block made up of spaced apart teeth 47 and 48 set within a circular recess 49. Lower locking block 44 comprises protruding cylindrical member 50 with surface spaced apart teeth 51 and 52. When assembled, as best seen in FIG. 4, protruding cylindrical member 50 fits rotatably part way into cylindrical recess 49. Also as best seen in FIG. 4, before failure of the layer 53 cap 56, inner hub 42 and outer hub 43 are fixed to and rotate with drive shaft 86. After failure of layer 53, outer hub 43 is free to rotate about drive shaft 86, at least until the locking blocks engage. When engaged as discussed further below, the teeth on the respective locking blocks loosely fit together to prevent relative rotational movement between the two locking blocks.

[0037] Surfaces 45 and 46 may have generally any configuration or slope, so long as the outer hub can move into the locking position upon failure of layer 53. The configuration shown has proven to be quite successful and functional.

[0038] One of skill in the art will recognize that there are many types of epoxy available that will work in this environment. By way of example, successful operation has been experienced with an epoxy known as “HP-120” by Loctight. A useful characteristic of epoxy, as used in the instant invention is that only a very thin layer is required to hold the hubs together, and when it fails, the material all but disappears. This feature is quite valuable and important from a practical point of view in that the cleanliness of shear failure promotes quick operation of the brake. Devices such as that shown in the '662 patent listed above would, upon failure of the shear device, present metallic debris that would likely interfere with, and “jam” up the brake disclosed herein.

[0039] While epoxy is disclosed as the preferred bonding or shear medium, there are other alternatives. For example, rubber, plastic, solder, brazing and the like could all be used. It is worthwhile to note that in the environment of a cutterhead, mechanical shear pins or similar fitted shear mechanisms tend to fail prematurely due to the fatigue experienced from the vibration and alternating stress inherent in the application.

[0040]FIG. 6 is an exploded cross-sectional view of the primary elements making up the preferred embodiment of the invention and is included herein for further clarity. The Belleville washers (springs) 54 and 55 are mounted in an opposing manner and shown here in their compressed state, but it can be clearly seen that when assembled the washers will exert a downward bias on outer hub 43. Thus, if the epoxy bond between the inner and outer hubs 42, 43 is broken, outer hub 43 will be biased to move downwardly.

[0041]FIGS. 7 and 8 are also presented for additional clarity. FIG. 7 is a top plan view of lower locking block 44 taken along line 7-7 of FIG. 6. Looking at this view and FIG. 9 it can be seen that there are raised areas, or teeth, 51 and 52 extending partially around the circumference of the protruding cylindrically shaped portion 50, forming lower interstitial areas 57. FIG. 8 shows that outer hub 43 includes a portion 59 which is referred to as the upper locking block. Upper locking block 59 can be either integral with outer hub 43, as shown here, or formed as a separate piece and affixed to outer hub 43. Teeth 47 and 48 are spaced apart, forming lower interstitial areas 58. Interstitial areas 57 and 58 are larger circumferentially than the respective teeth that are to be engaged therein. This difference in size is to allow time and space for the full engagement of the locking blocks upon failure of the shear layer. When assembled, the teeth of the two locking blocks are separated; however, if the epoxy bond fails, i.e., the shear mechanism fractures, outer hub 43 (and because they are integral, upper locking hub 59) drops into lower locking block 44—teeth 51 and 52 drop into spaces 58, and teeth 47 and 48 go into spaces 57. The two locking blocks are thus “locked” together, stopping the rotation of outer hub 43 (and thus disc member 80 and knives 82). Lower locking block 44 is fixed in place, i.e., does not rotate and thus provides solid support for the locking function.

[0042] The number and configuration of teeth on the locking blocks can, of course, vary depending upon several factors such as the harness of the base materials, the speed of rotation, the mass of the cutterhead, and the timing required to stop the relative movement of the components. In addition, for similar reasons, it may be appropriate or beneficial to harden the teeth, add cushioning material to the interfering faces and/or add friction-reducing materials to the contacting surfaces.

[0043] Referring now to FIGS. 3-5, inner hub 42 is detachably splined onto a drive shaft 86 having an integral driven gear 77 positioned within the oil reservoir 84. Inner hub 42 is affixed to outer hub 43 by a layer of epoxy 53 which, as described above, serves as a shearing device. A disc member 80 is detachably connected to outer hub 43 by bolts 81 so as to be rotatable therewith (and thus knives 82). The drive shaft 86 is rotatably supported by a bearing block 78 detachably mounted to the cutterhead module housing 41 by bolts 79. The bearing block 78 seals an opening in the top of the housing 41 through which the driven gear can be extracted from the oil reservoir 84.

[0044] When the cutterhead engages a fixed object of sufficient mass or rigidity to generate a shearing force on layer 53, adequate to cause failure thereof, the inner and outer hubs 42, 43 will separate and outer hub 43 will drop. In the preferred embodiment, outer hub 43 is biased downwardly by means such as Belleville washers 54 and 55 working between cap 56 and outer hub 43. FIG. 5 shows this change in position of outer hub 43 (has moved downwardly on shaft 86) in exaggerated form for illustrative purposes.

[0045] The drive mechanism 75 in each cutterhead module 40 is coupled to the other cutterhead module drive assemblies 85 by a transfer shaft 94 that passes through the spacer module 32, as best depicted in FIG. 2. The transfer shaft 89 is splined at each opposing end thereof to be finally received within either of the hubs 89, 90 to transfer rotational power thereto.

[0046] Referring now to the configurations of utilization of the cutterbar 30 as depicted in FIG. 1, it can be seen that the drive mechanism 75 in a disc mower 10 receives rotational power from a gearbox 17 that is supported adjacent the inboardmost cutterhead module 40. Accordingly, the drive assembly 85 is connected directly to the output shaft (not shown) of the gearbox 17. The transfer of rotational power to the remaining cutterhead modules 40 proceeds as described above.

[0047]FIGS. 10 and 11 show an important alternative structure to the brake. In the brake described above, the outer hub drops down to allow the locking blocks to engage. A potential problem with this arrangement is that crop materials and debris can build up on the lower part of the structure and potentially hinder or interfere with the quick engagement of the brake. While one could take steps and develop structure to reduce or prevent the build up of materials, the structure shown in FIGS. 10 and 11 eliminates the potential problem by causing the outer hub to move upward on the drive shaft 86 to engage the brake.

[0048] Inner hub 110 is connected to outer hub 112 by shear bolt 114. Of course, instead of a shear bolt(s) a frangible layer as taught above could also be used as the shear device. Between the inner hub 110 and outer hub 112 is a spring 116, in this embodiment a known wave spring, biasing the outer hub upwardly relative to inner hub 110 which is fixed to shaft 86 by, for example, splines. Annular lip 118 on cap 117 forms a stop against further upward movement of inner hub 110. Thus, upon failure of the shear device, outer hub 112 moves upwardly away from inner hub 110 under the bias of spring 116 until it engages lip 118 of cap 117.

[0049] Though the interlocking configuration of the locking blocks could be essentially the same as that described and shown in earlier Figs., this embodiment shows a slight modification. Lower locking block 120 has an annular recess 122 into which flange 124 of outer hub 112 fits. As best seen in FIG. 11, lower locking block 120 has spaced apart annular lips 126 forming annular openings 128 which matingly match teeth (not shown) on flange 124 such that the teeth mate with openings 128 to lock outer hub 112, when the shear device fails, in the manner described above. The dimensional relationship among the component parts is such that lip 118 stops upward movement of outer hub 112 at the same time the teeth on flange 124 move into openings 128 and engage the edges 130 of openings 128.

[0050] It will be understood that changes in the details, materials, steps and arrangements of parts which have been described and illustrated to explain the nature of the invention will occur to and may be made by those skilled in the art upon a reading of this disclosure within the principles and scope of the invention. The foregoing description illustrates the preferred embodiment of the invention; however, concepts, as based upon the description, may be employed in other embodiments without departing from the scope of the invention. Accordingly, the following claims are intended to protect the invention broadly as well as in the specific form shown. 

Having thus described the invention, what is claimed is:
 1. A disc cutterbar having a support member with a cutterhead affixed thereto, said cutterhead including a disc member carrying a knife to sever standing crop material, said disc member having a mounting hub provided with a central mounting hole; said mounting hub comprised of an inner hub fitted within an outer hub with said shear device operable between said inner and outer hubs, said disc member drivingly connected to said outer hub; a drive including a drive shaft received in said mounting hole for rotation of said disc member about an upright axis; said inner hub being circular in plan view and coaxially affixed to said drive shaft, said outer hub has a central circular opening into which said inner hub is fitted; a shear device operable between said drive shaft and said disc member such that torque is transferred from said drive shaft through said mounting hub and shear device to said disc member; and a brake associated with said shear device whereby upon failure of said shear device said brake stops the rotation of said disc member.
 2. The disc cutterbar of claim 1 wherein: said brake stops the rotation of said disc member in one revolution or less of said disc member after failure of said shear device.
 3. The disc cutterbar of claim 2 wherein: said brake is comprised of an upper locking block on the bottom of said outer hub and a lower locking block non-rotatably affixed to said support member, said upper and lower locking blocks being coaxial with said drive shaft and spaced apart from each other along the axis of said drive shaft.
 4. The disc cutterbar of claim 3 wherein: said drive shaft extends through said outer hub and said upper and lower locking blocks, and said lower locking block is movable along said drive shaft upon failure of said shear device; and said upper and lower locking blocks include generally mating teeth, whereby failure of said shear device causes said inner and outer hubs to separate, allowing said outer hub to move on said drive shaft toward said lower locking block and said teeth on said upper and lower locking blocks to engage, stopping the rotation of said outer hub.
 5. The disc cutterbar of claim 4 wherein: said hubs and locking blocks are held in position on said drive shaft by a cap member affixed to said drive shaft; and said outer hub is biased toward said lower locking block by a spring means operating between said cap member and said outer hub.
 6. The disc cutterbar of claim 5 wherein: said spring means is a pair of opposing Belleville washers fitted on said drive shaft.
 7. The disc cutterbar of claim 5 wherein: said inner and outer hubs have co-operating surfaces with a layer of frangible material bonding said co-operating surfaces together, said co-operating surfaces and layer of frangible material comprising said shear device.
 8. The disc cutterbar of claim 7 wherein: said co-op e rating surfaces are no t parallel to the axis of said drive shaft and are in interference with each other.
 9. The disc cutterbar of claim 8 wherein: said layer of frangible material is comprised of an epoxy.
 10. A disc cutterbar having a support member with a plurality of transversely spaced cutterheads affixed thereto, each said cutterhead drivingly connected to a vertical driveshaft having a mounting hub with a disc member rotatably supported thereon, said disc member carrying a knife to sever standing crop material; said plurality of cutterheads arranged and mounted such that they rotate on their respective vertical driveshafts in a timed non-interference relationship one with the other, and said hub is comprised of an inner hub fitted within an outer hub with a shear device operable between said inner and outer hubs; said disc member drivingly connected to said outer hub.
 11. The disc cutterbar of claim 10 wherein: said inner hub is circular in plan view and coaxially affixed to said drive shaft, and said outer hub has a central circular opening into which said inner hub is fitted; and said inner and outer hubs have co-operating surfaces with a layer of frangible material bonding said co-operating surfaces together, said co-operating surfaces and layer of frangible material comprising said shear device.
 12. The disc cutterbar of claim 11 wherein: said co-operating surfaces are not parallel to the axis of said drive shaft and are in interference with each other.
 13. The disc cutterbar of claim 12 wherein: said layer of frangible material is comprised of epoxy.
 14. The disc cutterbar of claim 10 further including a brake associated with said shear device whereby upon failure of said shear device said brake stops the rotation of said disc member.
 15. The disc cutterbar of claim 14 wherein: said brake is comprised of an upper locking block on the bottom of said outer hub and a lower locking block affixed to said support member, said upper and lower locking blocks being coaxial with said drive shaft and spaced apart from each other along the axis of said drive shaft.
 16. The disc cutterbar of claim 15 wherein: said drive shaft extends through said outer hub and said upper and lower locking blocks; and said upper and lower locking blocks include generally mating teeth, whereby failure of said shear device caused said inner and outer hubs to separate, allowing said outer hub to drop down said drive shaft and said teeth on said upper and lower locking blocks to engage, stopping the rotation of said outer hub.
 17. The disc cutterbar of claim 16 wherein: said hubs and locking blocks are held in position on said drive shaft by a cap member affixed to said drive shaft; and said outer hub is biased toward said lower locking block by a spring means operating between said cap member and said outer hub.
 18. The disc cutterbar of claim 17 wherein: said spring means is a pair of opposing Belleville washers fitted on said drive shaft.
 19. A disc cutterbar comprising: a support member with a cutterhead affixed thereto; a disc member affixed to said cutterhead and carrying a knife to sever standing crop material; a mounting hub in said disc member provided with a central mounting hole and including an inner hub fitted within an outer hub and containing said central mounting hole; a drive including a drive shaft received in said mounting hole for rotation of said disc member about an upright axis; a shear device associated with said mounting hub such that torque is transferred from said upright drive shaft through said shear device to said disc member; said shear device operable between said inner and outer hubs, said disc member drivingly connected to said outer hub; and a brake associated with said outer hub whereby upon failure of said shear device, said outer hub ceases to be driven, and said brake stops the rotation of said outer hub.
 20. The disc cutterbar of claim 19 wherein: said inner hub is circular in plan view and coaxially affixed to said drive shaft, and said outer hub has a central circular opening into which said inner hub is fitted; and said inner and outer hubs have co-operating surfaces with a layer of frangible material bonding said co-operating surfaces together, said co-operating surfaces and layer of frangible material comprising said shear device.
 21. The disc cutterbar of claim 20 wherein: said co-operating surfaces are not parallel to the axis of said drive shaft and are in interference with each other.
 22. The disc cutterbar of claim 21 wherein: said layer of frangible materials is comprised of epoxy.
 23. The disc cutterbar of claim 20 wherein: said brake is comprised of an upper locking block on the bottom of said outer hub and a lower locking block non-rotatably affixed to said support member, said upper and lower locking blocks being coaxial with said drive shaft and spaced apart from each other along the axis of said drive shaft.
 24. The disc cutterbar of claim 23 wherein: said drive shaft extends through said outer hub and said upper and lower locking blocks; and said upper and lower locking blocks include generally mating teeth, whereby failure of said shear device causes said inner and outer hubs to separate, allowing said outer hub to move on said drive shaft toward said lower locking block and said teeth on said upper and lower locking blocks to engage, stopping the rotation of said outer hub.
 25. The disc cutterbar of claim 24 wherein: said hubs and locking blocks are held in position on said drive shaft by a cap member affixed to said drive shaft; and said outer hub is biased toward said lower locking block by a spring means operating between said cap member and said outer hub.
 26. The disc cutterbar of claim 25 wherein: said brake stops the rotation of said disc member in one revolution or less of said disc member after failure of said shear device.
 27. The disc cutterbar of claim 26 wherein: said layer of frangible materials is comprised of epoxy. 